The tenor guitar or four-string guitar is a slightly smaller, four-string relative of the steel-string acoustic guitar or electric guitar. The instrument (in its acoustic form) was developed so that players of the four-string tenor banjo could double on the guitar. Later, solid-body electric models were also produced.

Tenor guitars are four stringed instruments normally made in the shape of a guitar, or sometimes with a lute-like pear shaped body or, more rarely, with a round banjo-like wooden body. They can be acoustic and/or electric and they can come in the form of flat top, archtop, wood-bodied or metal-bodied resonator or solid-bodied instruments. Tenor guitars normally have a scale length (from bridge to nut) of between 21 and 23 inches.

The earliest origins of the tenor guitar are not yet fully clear but it now seems very unlikely that a true four-stringed guitar-shaped tenor guitar appeared before the late 1920s. Gibson built the tenor lute TL-4 in 1924, which had a lute-like pear-shaped body, four strings and a tenor banjo neck. It is possible that similar instruments were made by other makers such as Lyon and Healy and banjo makers, such as Bacon. In the same period, banjo makers, such as Paramount, built transitional round banjo-like wood-bodied instruments with four strings and tenor banjo necks called tenor harps. From 1927 onwards, the very first true wood-bodied acoustic tenor guitars appeared as production instruments made by both Gibson and Martin.